1. Technical Field
The present invention relates generally to motor vehicle emissions testing and, more particularly, to a computer controlled sealed housing evaporative determination (SHED) testing apparatus and method for automatically testing motor vehicle evaporative emissions under a number of testing schedules.
2. Discussion
In recent years, motor vehicle manufacturers have greatly reduced the levels of hydrocarbons, e.g., carbon monoxide, carbon dioxide, etc., and other gasoline and diesel powered motor vehicle emissions in response to increased governmental regulations aimed at preserving and protecting the environment. In addition to the commonly known tailpipe emissions, i.e., the exhaust gases produced during the combustion process of the motor vehicle engine, there are also evaporative emissions. That is, a motor vehicle produces emissions while simply sitting parked due to evaporation of oil, fuel and other fluids which are common to motor vehicles.
In this regard, modern motor vehicles incorporate sealed fuel and lubrication systems which often include charcoal canisters and the like for collecting vapors produced as a result of evaporation of these hydrocarbon based fluids. The conventional sealed fuel and lubrication systems typically retain the vapors for later burning in the vehicle engine when the vehicle is running. Still, not all of the vapors can be collected, and those which are not collected are generally classified as evaporative emissions.
Governmental regulations such as those promulgated by the Federal Environmental Protection Agency (EPA) and the California Air Resource Board (CARB) often establish strict limitations on the amount of emissions, both exhaust gas and evaporative, that a motor vehicle may produce. Modern day regulations also require that a motor vehicle manufacturer test and certify that the vehicles manufactured and sold conform to these regulations. The allowable amounts of emissions are often measured as parts per million (ppm) of a total sample of air collected. Thus, vehicle manufacturers require sophisticated and highly accurate testing equipment for performing the necessary emissions tests. Testing for evaporative emissions is typically conducted according to what is commonly known as a sealed housing evaporative determination (SHED) test.
Early evaporative emissions regulations mandating SHED tests generally only required that a vehicle be operated for a given period of time and then the amount of evaporative emissions produced during a set time period thereafter (usually one hour) be measured. In the past, this type of SHED test was simply conducted by operating the vehicle on a test track or chassis dynamometer and then placing the vehicle in a sealed structure for a required time period. The amount of emissions produced was simply the difference in hydrocarbon levels measured at the beginning of the test and at the end of the test.
Modern regulations recognize that a motor vehicle produces emissions as a result of evaporation of fluids due to heating and cooling conditions of the surrounding environment. Therefore, such modern regulations require measuring evaporative emissions produced by a vehicle over the course of several days and in response to changing temperature conditions. That is, the vehicle must be cycled through a number of temperature changes over the course of several days so as to simulate what the vehicle might experience, due to heating by the sun during the day and cooling during the evening and night, while parked for several days. Such testing requirements are known as variable temperature SHED tests and pose a number of difficulties.
One of the difficulties with performing variable temperature SHED tests in accordance with modern regulations lies in controlling the temperature and pressure within the sealed SHED testing structure. Changing the SHED temperature can be accomplished by providing a heating unit and a cooling unit with controls for maintaining the SHED structure at the appropriate temperature. However, changes in the air temperature cause corresponding changes in the volume and hence changes in the pressure of the air within the SHED structure. Air pressure changes within the SHED structure can cause a number of adverse affects on the results provided by the SHED test. For example, changes in air pressure affect the performance of the vehicle emission control systems. Also, leaks often remain in the SHED test housing in spite of the fact that every effort is made to completely seal the SHED structure housing. Pressure differences between the interior of the SHED structure housing and the outside environment encourage a migration of air either into or out of the SHED structure through any leaks, thus affecting the accuracy of the test results. Therefore, it is desirable to control the pressure changes in order to maintain a pressure difference between the interior of the SHED structure and the surrounding outside environment as near zero as possible.
Several testing systems have been proposed to accomplish the desired pressure control. Recognizing that the pressure change is a response to an attempt to change the volume of the air within the SHED structure as a result of changing air temperature, one conventional system incorporates a moving roof on the SHED structure housing. The moving roof is in the form of a mechanical operated bellows which causes the roof to rise and fall in accordance with changes in temperature, and therefore volume, of the air within the SHED structure. In this manner, the volume of the SHED structure is made to correspond to the volume of air within the structure for any given temperature. The volume of the SHED structure is therefore determined by the position of the roof, and a calculation of the evaporative emissions can be made based upon the known volume of air. The moving roof approach, however, proves to be prohibitively expensive and difficult to maintain as it requires complex mechanical and control elements.
Similarly, another conventional system incorporates a bladder bag in place of the moving roof. As pressure within the SHED structure housing changes, air is pumped into or released from the bladder bag. Like the movable roof approach, the calculation of the evaporative emissions can be made based upon the known volume of air including the air contained in the bladder bag. This system, however, tends to interfere with the uniform mixing of the air within the SHED structure which may adversely affect the test results.
It is therefore one object of the present invention to provide for an apparatus and method for conducting SHED tests which provides accuracy in accordance with required governmental regulations.
Another object of the present invention is to provide a SHED testing apparatus and method which is operable to perform extended duration variable temperature SHED tests automatically and without the need for operator intervention during the test.
Yet, a further object of the present invention is to provide for an enhanced method for carrying out the SHED testing operation so as to accurately measure evaporative emissions of a motor vehicle in a fixed structure housing over a series of temperature changes so as to maintain a substantially constant pressure differential of zero and without the need for a mechanically expandable air storage compartment.